How to compare protein creatinine ratio (PCR) and albumin creatinine ratio (ACR)?

Comparing Protein-Creatinine Ratio (PCR) and Albumin-Creatinine Ratio (ACR)

Use ACR as your primary test for screening and monitoring kidney disease, as it is more sensitive for detecting early kidney damage and is the preferred measure in current KDIGO guidelines, but recognize that PCR measures total protein and may be needed when non-albumin proteinuria is suspected. 1

Direct Conversion Between PCR and ACR

The relationship between PCR and ACR is not linear and varies with the degree of proteinuria 1, 2:

• At normal to mildly increased levels:

  • ACR <30 mg/g corresponds to PCR <150 mg/g 1
  • The albumin-to-protein ratio is approximately 30% or less in this range 3

• At moderately increased levels:

  • ACR 30-300 mg/g corresponds to PCR 150-500 mg/g 1
  • The albumin-to-protein ratio increases progressively 3

• At severely increased levels:

  • ACR >300 mg/g corresponds to PCR >500 mg/g 1
  • The albumin-to-protein ratio reaches approximately 70% in severe proteinuria 3

You cannot simply multiply ACR by a fixed number to get PCR - the ratio changes depending on the severity of proteinuria 3, 2.

Key Differences in What Each Test Measures

ACR measures only albumin, which is the predominant protein filtered in most kidney diseases, while PCR measures total urinary protein including albumin plus tubular proteins, immunoglobulins, and other plasma proteins 4, 2.

• ACR is more sensitive for detecting early kidney damage at lower levels of proteinuria 4, 2
• ACR can be standardized using immunochemical techniques, making it more reliable across laboratories 4
• PCR may detect non-albumin proteinuria that could indicate tubular defects, tubulointerstitial disease, or paraprotein disorders 4, 2

When to Use Each Test

For initial screening and monitoring, use ACR 1:

• Diabetic kidney disease screening and monitoring 4
• Early CKD detection in at-risk patients 4
• Annual monitoring in established CKD 4

Consider PCR when:

• Non-albumin proteinuria is suspected (tubular disorders, myeloma) 4, 2
• ACR is disproportionately low compared to clinical suspicion 2
• Evaluating patients not on ACE inhibitors or ARBs, as non-albumin proteinuria is proportionally higher in these patients 2

Clinical Performance Comparison

PCR outperforms ACR for predicting clinically significant proteinuria thresholds 2:

• At the 1 g/day threshold: PCR has higher sensitivity (94% vs 79%) but lower specificity (88% vs 95%) than ACR 2
• To achieve comparable 94% sensitivity, ACR threshold must drop to 17.5 mg/mmol, which reduces specificity to only 70% 2

Both tests correlate well with 24-hour urine protein, but PCR shows stronger correlation (Spearman's rho 0.91 vs 0.84 for ACR) 2.

Important Factors Affecting Interpretation

Age and sex significantly affect the relationship between PCR and ACR 2:

• Men generally have a higher albumin-to-protein ratio than women 3, 2
• To achieve 95% sensitivity: men <49 years require PCR of 65 mg/mmol, while women >79 years require PCR of 179 mg/mmol 2

Muscle mass extremes can distort both ratios since creatinine excretion varies with muscle mass 1, 5:

• Low muscle mass may overestimate proteinuria using ratio-based methods 5
• High muscle mass may underestimate proteinuria 5

Other factors causing variability 1:

• Urinary tract infection, fever, and heart failure cause transient elevations 4
• Exercise increases both albumin and protein excretion 1
• Hematuria and menstruation falsely elevate results 1

Practical Collection and Confirmation

First morning void is preferred for both ACR and PCR as it correlates best with 24-hour collections and has lowest variability 1, 4.

Confirm abnormal results 1:

• Confirm ACR ≥30 mg/g on a random sample with a subsequent first morning void 1
• Use quantitative laboratory measurement to confirm positive reagent strip results 1
• Repeat testing within 3 months before making clinical decisions 4

Common Pitfalls to Avoid

• Do not assume a fixed conversion factor between ACR and PCR - the relationship is non-linear and changes with proteinuria severity 3, 2
• Do not rely solely on ACR in non-diabetic CKD without considering that significant non-albumin proteinuria may be missed 2
• Do not ignore the effect of ACE inhibitors/ARBs - these medications reduce non-albumin proteinuria proportionally more than albumin, affecting the ACR:PCR relationship 2
• Avoid using the outdated term "microalbuminuria" - use the KDIGO categories A1, A2, A3 instead 1, 4